An annular gasket is interposed between the components to be sealed of an opening such as between the contacting faces of the cylinder block and the oil pan of an internal-combustion engine like automobile engines and a joined portion of several piping. By fastening these components to be sealed, the gasket is interposed by pressure to achieve sealing. Such a gasket is manufactured by punching and processing a rubber sheet, cellulose fiber sheet, cork, cork rubber, or a sheet-like base material which is integrally formed on both faces of a metal base plate with an elastic material such as rubber to be integrated, thereby forming a predetermined annular gasket.
When the sheet-like base material is punched and processed as mentioned above, the inner portion of annular object is discarded as waste. In particular, when an opening area is large and the rate of the inner side portion of the annular object is large like the gasket for oil pan, or the gasket for piping of a large caliber, this waste portion is increased, thereby deteriorating the production yield. The patent document 1 and the patent document 2 disclose a production method of an annular gasket in order to solve such productivity problems in which a sheet-like base material is punched and processed into a plurality of gasket component members and the component members are joined.
The gasket produced by the method of the patent document 1 is made of a compressible gasket material containing much bubble or space therein. Recently, from viewpoints of toughness and durability what is called a metallic gasket has been used as the above-mentioned gasket for oil pan as disclosed in the patent document 2, such gasket is formed by integrating a covering material (seal layer) made of a soft elastic seal material such as rubber on both faces of a metal sheet like a steel plate.    [Patent document 1] JP-58-21142-B    [Patent-document 2] JP-2003-336743-A